What is the point of Dream World Project Difficulty Levels?

It seems to me that the 'higher' levels of difficulty ask for more contributions in the same time frame, but the reward is less. For example:
Level 1 corn project requires 3000 corn for 20 points (ignoring any bonus)
Level 2 corn project requires 9000 corn for 50 points.
It makes more sense to do 3 level 1 projects (60 points) than 1 level 2 project (50 points).
The only time the higher level gives a better reward is when there is a bonus available, which is not very often.
So if it is actually a higher difficulty, then why does it not always give a higher reward?
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